Original episode & show notes | Raw transcript
This document provides a detailed breakdown of the advanced concepts in exercise physiology and sports nutrition discussed in the Empirical Cycling Podcast episode featuring Tim Podlogar, a research fellow at the University of Birmingham and nutritionist for the Bora-Hansgrohe cycling team.
Fructose often has a negative reputation in public health discussions, but for athletes, it’s a powerful tool for performance and recovery.
In a sedentary context, high fructose consumption (e.g., from sugary drinks) can be problematic. Unlike glucose, which can be used by most cells in the body, fructose is metabolized almost exclusively in the liver. If the liver’s glycogen stores are already full, the excess fructose can be converted into fat (de novo lipogenesis), potentially contributing to metabolic issues like non-alcoholic fatty liver disease.
For an athlete, the context is completely different. High energy expenditure means the liver is rarely “full,” and fructose becomes a valuable, fast-acting energy source.
Dual-Transporter System: The human gut absorbs carbohydrates through different cellular transporters. Glucose and maltodextrin use the SGLT1 transporter, which has a saturation point of around 60-70 grams per hour. Fructose uses a separate transporter, GLUT5. By consuming both glucose/maltodextrin and fructose, athletes can bypass the single-transporter bottleneck and absorb significantly more carbohydrates—upwards of 120 grams per hour.
Liver Glycogen Replenishment: Fructose is exceptionally effective at replenishing liver glycogen, which is crucial for staving off hypoglycemia (bonking) and signaling energy availability to the body. Podlogar emphasizes its use in recovery and pre-race nutrition to ensure the liver is fully stocked.
Provides an Alternative Fuel (Lactate): In the liver, fructose is converted into glucose, glycogen, or lactate. This lactate can then be released into the bloodstream and used directly by working muscles as a high-octane fuel. Consuming fructose during exercise can raise blood lactate levels, not from muscular production, but from liver conversion, providing another ready-to-use energy substrate.
Optimal Ratio: Podlogar recommends a 1-to-0.8 ratio of maltodextrin-to-fructose for on-bike fueling and recovery, as this has been shown to maximize absorption and oxidation rates.
The podcast delves into the complexities of defining and testing physiological thresholds, moving beyond simplistic definitions.
This threshold marks the boundary between the moderate and heavy exercise intensity domains. It’s the point where aerobic metabolism is still dominant, but lactate begins to rise slightly above baseline levels.
How to Identify LT1:
Lactate Testing: A small, sustained increase in blood lactate (Podlogar suggests a 0.5 mmol/L increase from baseline).
Ventilatory Testing (VT1): The first point where breathing rate begins to increase without a corresponding disproportionate increase in the volume of air expired.
Practical Methods: The “talk test” (the highest intensity at which you can speak in full sentences comfortably) and monitoring heart rate drift. Over a long duration (2-4 hours), if your heart rate remains stable at a given power output, you are likely at or below LT1. If it steadily drifts upward, you are above it.
This is the boundary between the heavy and severe exercise domains. Above this point, metabolic homeostasis can no longer be maintained, and fatigue accumulates rapidly.
The Problem of a Single Definition: There is no single “gold standard” physiological event that defines this threshold perfectly for all athletes.
Maximal Lactate Steady State (MLSS): The highest intensity where blood lactate concentration remains stable. This requires multiple lab visits and can vary day-to-day.
Functional Threshold Power (FTP): Defined by time (e.g., 60 minutes) rather than a direct physiological state. It’s a practical performance metric but may not perfectly align with MLSS.
Critical Power (CP): A model derived from multiple all-out efforts of varying durations. It is thought to represent the boundary of muscular metabolic stability more closely than blood-based measures. However, its time-to-exhaustion can vary widely between individuals (from 15 to 40+ minutes).
The Key Takeaway: No single test is perfect. The most important thing is consistency in testing methods and understanding the limitations of the chosen model. For a coach, the host argues that the most practical definition is the inflection point in an athlete’s power-duration curve, as this represents the real-world boundary between sustainable and unsustainable efforts, regardless of the underlying physiology.
While VO2 max is a crucial determinant of endurance potential, Podlogar argues it is a poor descriptor of performance or training status in already elite athletes.
The Plateau Effect: An elite cyclist’s VO2 max will likely not change significantly year after year. It’s a physiological ceiling they have already reached.
The Efficiency Factor: The podcast highlights a key observation: you can have two athletes with the same elite VO2 max (e.g., 75 ml/kg/min), but one might have a threshold at 4.5 W/kg and the other at 5.3 W/kg. The difference isn’t their aerobic “engine size” (VO2 max) but their efficiency—how effectively they can translate that oxygen consumption into power at sub-maximal intensities.
A Better Metric: For this population, the second threshold (CP, MLSS, etc.) is a much better predictor of performance and a more sensitive marker of training adaptation. Improvements in performance for an elite rider come from pushing their sustainable power output closer to their VO2 max, not from increasing the VO2 max itself.
Can restricting carbohydrates around certain workouts enhance adaptation? The answer is highly dependent on context.
For time-crunched athletes (e.g., those with 10 hours/week), the total training stimulus is limited. In this case, performing some low-intensity, steady-state sessions with low carbohydrate availability (e.g., fasted in the morning) can act as an additional stressor, amplifying the cellular signaling (e.g., AMPK activation) that drives mitochondrial adaptation.
For high-volume athletes (20-30 hours/week), the training itself provides an enormous stimulus. The primary goal of nutrition is to support this training, not to add more stress.
For these athletes, high carbohydrate availability is crucial to:
Maintain high-quality training intensity.
Complete the prescribed volume.
Recover adequately between sessions.
Restricting carbohydrates would compromise training quality and recovery, leading to a net negative outcome. The risk of slipping into Relative Energy Deficiency in Sport (RED-S) is also significantly higher.
The Real Mechanism: Adaptation is not about “teaching the body to burn more fat.” It’s about creating cellular stress that stimulates mitochondrial biogenesis. The ultimate goal is to increase the total flux through the mitochondria, regardless of whether the fuel is fat or carbohydrate. An elite athlete like Tadej Pogačar has incredibly high fat oxidation rates because his mitochondrial density is immense from years of high-volume training, not because he follows a specific “fat-adaptation” diet.
Sodium Bicarbonate: It works primarily as an extracellular buffer. It soaks up hydrogen ions (H+) in the bloodstream, which increases the gradient for H+ and lactate to move out of the muscle cell. This helps to delay the drop in muscle pH, which is thought to impair enzyme function and muscle contraction. The main drawbacks are significant risk of gastrointestinal distress and water retention.
Beta-Alanine: It increases muscle carnosine levels. While commonly marketed as an intracellular buffer (working inside the muscle), Podlogar suggests a more likely mechanism is that carnosine improves the muscle’s sensitivity to calcium, which is critical for initiating muscle contraction.
Creatine: Traditionally used for strength and power, Podlogar makes a case for its use in endurance cycling. The goal is to increase functional muscle mass, which can raise absolute power output. This challenges the “lighter is always better” dogma. For a rider like Nairo Quintana, a higher body mass with more muscle could translate to a higher absolute wattage, making him more competitive on flat terrain and in time trials, potentially without harming his watts/kg on climbs.
Podlogar offers a sharp critique of the modern scientific publishing landscape, specifically targeting publishers like MDPI (Multidisciplinary Digital Publishing Institute).
The “Pay-to-Publish” Model: These journals operate on an open-access model where researchers pay a hefty fee (e.g., €2000-€3000) to have their article published.
The Conflict of Interest: The publisher’s business model is incentivized by the quantity of papers published, not the quality. This often leads to:
Lax Peer Review: The review process can be rushed and superficial to ensure a high acceptance rate.
Inflation of Low-Quality Science: The barrier to publication is lowered, flooding the literature with studies that may be poorly designed or statistically weak.
How to Be a Critical Consumer of Science:
Check the Journal’s Reputation: Be skeptical of papers from publishers known for this model (MDPI, Frontiers, etc.), although good papers can still be found in them.
Investigate the Authors: Are they from a well-respected university or research group? Do they have a track record of publishing in traditional, high-impact journals?
Ask “Why Here?”: A study published in a lower-tier journal may have been rejected by more rigorous journals. Consider why that might be.